1. Field of the Invention
The invention relates to an automatic clearance adjuster for a brake and more specifically to such an adjuster having a torque limiting clutch for limiting clearance decreasing adjustment during the brake stroke in one direction and a roller clutch for preventing clearance increasing adjustment of the brake during the brake stroke in the other direction.
2. Description of the Prior Art
Automatic clearance adjusters employing means to prevent over adjustment of a vehicle brake due to elastic deformation of the brake assembly are well known. Such adjusters are said to be of the "clearance sensing type" which distinguish between the two parts of any brake apply or release stroke by sensing the relatively low braking forces required to move the brake shoes when the shoes are out of contact with the brake drum and the relatively high braking forces when the shoes are in contact with the drum. An adjusting mechanism in the adjuster automatically makes a clearance decreasing adjustment when the low force part of the brake stroke exceeds a predetermined length. The adjusting mechanism is blocked out or prevented from making a clearance decreasing adjustment during the high force part of the brake stroke to prevent over adjustment of the brake. The high force part of the brake stroke is often larger than the first part due to the high forces causing deformation in the brake assembly and therefore can quickly over adjust the brake if the adjusting mechanism is not blocked out.
One prior art brake adjuster, of the clearance sensing type, employs a disengaging clutch which physically disengages an adjusting mechanism in response to the high braking forces during both the brake apply and release strokes. This particular brake adjuster also includes an expanding type spring clutch which slips during the low force part of the brake apply stroke for preventing clearance adjustment and which resists slippage during the low force part of the brake release stroke for allowing clearance adjustment should the length of the low force part of the release stroke exceed a predetermined amount.
Another prior art brake adjuster of the clearance sensing type employs a torque limiting spring clutch having a high torque transmitting capacity in one driven direction and a low capacity in the other drive direction. The clutch slips in its high torque direction in response to the high braking forces during the brake apply stroke to prevent adjustment while the shoes are in contact with the drum. In this particular brake adjuster, clearance adjustment is made during the brake apply stroke should the low force part of the brake apply stroke exceed a predetermined amount. During the brake release stroke, the spring clutch slips in its low torque direction, first due to the high braking forces and then during the low force part of the stroke due to frictional forces on the adjusting mechanism caused by a Belleville spring.
While these adjusters provided a significant advance in the automatic adjust art, the clutches employed therein to control clearance adjustment require a rather high degree of quality control in manufacture and assembly.